Currently, crystalline silicon has the largest market share in the photovoltaics (PV) industry, accounting for over 80% of the overall PV market share. Although going to thinner crystalline silicon solar cells is long understood to be one of the most potent ways to reduce PV cost (because of the relatively high material cost of crystalline silicon wafers used in solar cells as a fraction of the total PV module cost), it is fraught with the problem of mechanical breakage due to the thin and large substrate sizes, and also to some extent that of light trapping in a thin structure (since silicon is an indirect bandgap semiconductor material). The requirement of high mechanical yield and reduced wafer breakage rate is further problematic with the realization that for cost-effectiveness, the yields in PV manufacturing factories must be very high. On a standalone crystalline silicon solar cell (without support), going even somewhat below the current thickness range of 140-250 microns starts to severely compromise mechanical yield during manufacturing. Thus, any solution to process very thin solar cell structures must either be fully supported by a host carrier throughout the cell process or use a novel self-supporting, standalone, substrate with an accompanying structural innovation.
Innovative manufacturing processes to produce solar cells at low costs using thin-film semiconductor substrates (TFSSs) as the active material have been disclosed in related applications. In some embodiments, the TFSSs comprise crystalline semiconductor (more specifically silicon in some embodiments). This technology platform produces solar cells from re-usable semiconductor templates. In certain embodiment, the surfaces of the templates have a periodic array of pre-structured 3-dimensional (3-D) structures. Examples of the 3-D TFSS based solar cells include but are not limited to prism honey-comb and inverted pyramidal cavities, which are described in earlier applications having common inventorship and/or ownership. The following are examples of related applications.
U.S. Patent Publications US2008/0157283 A1, US2008/0289684 A1, US2010/0148318 A1, US2010/0300518; U.S. patent application Ser. No. 13/057,104; PCT Application Serial Nos. PCT/US10/60591, PCT/US10/62614.
As disclosed by the above documents, the epitaxial substrate is grown on top of a reusable template and is subsequently dislodged. In one embodiment, the template and substrate each comprise monocrystalline silicon. Afterwards, the reusable template may be reused several times, with or without reconditioning, to grow more epitaxial substrates. The reusable template has a planar top surface or a top surface with pre-formed 3-D microstructures, such as hexagonal honey-comb cavities or inverted pyramidal cavities. The releasing of the 3-D TFSS is achieved with an interim sacrificial layer. The sacrificial layer should satisfy two important criteria. First, it needs to transfer the information on crystallinity from the reusable template to the epitaxial layer. Second, it should be able to be removed selectively compared to the substrate and the reusable template. One specific embodiment of the sacrificial layer is porous silicon, whose porosity can be modulated to achieve both the aforementioned critical functions.
As disclosed by the above documents, the planar or 3-D TFSSs are made and released from one-side, i.e., the top surface of a reusable template. In other words, one solar substrate is made from a reusable template in each of its reuse cycles. In those disclosures, the template and substrate making equipment, and the porous silicon forming and epitaxial silicon growing equipment is capable of processing on only one side of the silicon template. The equipment is capable of processing multiple wafers at a time in a batching processing mode, but only one side of each template is used.
Solar cell manufacturing requires much higher productivity with much lower costs compared with semiconductor processes. Therefore, there it may be advantageous to develop manufacturing processes and equipment capable of making thin semiconductor substrates from both sides of a reusable template simultaneously.